Polysiloxane compositions are used in various industries because of their excellence in heat resistance, cold resistance, weather resistance, light resistance, chemical stability, electrical characteristics, flame retardancy, water resistance, transparency, colorability, anti-adhesive properties, and anti-corrosive properties. In particular, compositions including a polyhedral polysiloxane are known for their greater excellence in heat resistance, light resistance, chemical stability, low dielectricity and the like due to their specific chemical structures. Therefore, these compositions are expected to be used in various applications.
Some of the applications of polyhedral polysiloxanes are intended for the application as encapsulants for optical elements. For example, Patent Literature 1 discloses a polysiloxane composition having a polyhedral skeleton, which contains a polyhedral polysiloxane resin containing at least two oxetanyl groups, an aliphatic hydrocarbon containing at least one epoxy group, and a cationic polymerization initiator. This material has high refractive index and high light extraction efficiency. However, since the disclosed polysiloxane composition contains oxetanyl and epoxy groups, its heat resistance and light resistance are low. Therefore, the composition especially has difficulty in applications requiring high heat and light resistance, such as white LEDs.
In order to solve such a problem, for example, Patent Literature 2 discloses limiting the glass transition temperature of a polyorganopolysiloxane containing an epoxy group, to improve the heat and light resistance. This material is also thought to have less tendency to crack even after a thermal shock test. However, this material still has difficulty in applications requiring high heat and light resistance, such as white LEDs, and its crack resistance has not reached a sufficient level.
In addition, polysiloxane compositions generally have a problem of low gas-barrier properties in spite of their excellence in other properties. This problem causes another problem in which if a polysiloxane composition with low gas-barrier properties is used as an encapsulant of a reflector, the reflector turns black due to sulfide. In order to deal with this problem, for example, Patent Literature 3 discloses coating metal members with an acrylic resin having high gas-barrier properties before encapsulating them with a silicone resin. However, the silicone resin itself used in this technique has low gas-barrier properties, and this technique also has a problem with productivity in that it requires additional work such as an additional process of encapsulating with a silicone resin after coating with an acrylic resin.
Patent Literature 4 discloses a composition which includes a modified polyhedral polysiloxane. This composition is excellent in moldability, transparency, heat and light resistance, and adhesion, and thus can be used as an encapsulant for optical use. However, there is still scope for further improvement in gas-barrier properties.
As described above, although there are some disclosures of materials including polysiloxanes, there has been found no disclosure of a material having excellent gas-barrier properties while keeping high heat and light resistance. Accordingly, developing a new material is desired.